[CD-554] default styles for field_group details/summary elements

[rupl]

CD-554

Types of changes

• ✔️ Improvement (non-breaking change which iterates on an existing feature)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Security update (dependency updates, or to fix a vulnerability)

Description

When collapsible <details> /<summary> elements are present, they didn't look clickable. Now they do, and have a background color to show how far they extend.

Steps to reproduce the problem or Steps to test

1. Load a form that uses the field_group module (AI Summarization site has one)
2. Interact with the widget via mouse or keyboard.

Impact

CSS only. No impact.

PR Checklist

• I have followed the Conventional Commits guidelines.